A radio frequency identification system (RFID system) has been known. The RFID system is configured to read information from a RFID tag using a reader/writer. The RFID system sends a radio frequency signal of about 1 W (watt) to the RFID tag which is distantly-positioned from the RFID system and receives a response signal from the RFID tag. The channel used for sending and receiving radio signals between the RFID system and the RFID tag may be in the UHF band (860 to 960 MHz). In Japan, radio frequencies ranging from 952 to 954 MHz are used as the channel. The communication distance between the RFID system and the RFID tag is about 3 to 10 m, depending on the antenna gain of the RFID tag used, the operating voltage of a radio IC chip used, the antenna gain of the reader/writer used and the surrounding environment. The RFID tag includes an antenna and the IC chip (about 0.5 mm square) which is electrically coupled with a feed point of the antenna without mounting a specific matching circuit. In the RFID tag, an antenna pattern is formed on a transparent film sheet by printing, etching or the like.
The IC chip of the RFID tag may be equivalently expressed using a parallel circuit of an internal resistance Rc (for example, 1700 Ω) and a capacitance Cc (for example, 1.0 pF). Likewise, the antenna of the RFID tag may be equivalently expressed using a parallel circuit of a radiation resistance Ra (for example, 2000 Ω) and an inductance La (for example, 30 nH). Owing to parallel connection of the IC chip with the antenna, a resonance will be generated by the capacitance Cc and the inductance La to establish impedance matching at a desirable resonance frequency fo (for example, 953 MHz). As a result, the RFID tag is allowed to obtain a maximum received power. The resonance frequency fo is expressed as follow:
      f    o    =            1              2        ⁢        π        ×                              (                                          L                a                            ×                              C                c                                      )                                1            /            2                                .  
There is also known an electromagnetic wave transmission sheet which includes a meshed electrode to be usable for the RFID system. The sheet has a width dimension to almost equal to the integral multiple of a half of the wave length of the electromagnetic wave which travels along the surface of the sheet in a direction orthogonal to the direction of the width. Due to the width dimension, the sheet may produce a resonance of the electromagnetic wave in the direction orthogonal to the travelling direction. The electromagnetic wave transmission sheet has a three-layered structure: the meshed electrode, a flat plate electro conductive layer, and a dielectric layer which is sandwiched by the others. The structure is understood to contribute generation of the electromagnetic wave in a certain distance above from the sheet. As an application of the electromagnetic wave transmission sheet, Japanese Laid-open Patent Publication No. 2010-114696 has disclosed a RFID system for managing goods stocked on a shelf. The system includes a reader/writer and the electromagnetic wave transmission sheet which are electrically coupled each other with a coaxial cable. The electromagnetic wave transmission sheet is used as an antenna and disposed within the shelf to detect an RFID tag stuck on a peace of the goods to be managed by the RFID system. The RFID system has an advantage that a problem is prevented from erroneous detection of an RFID tag, which is pasted on goods not managed by the system, caused by unexpected transmission range of the electromagnetic wave from an antenna.
However, the conventional sheet has a problem that the detection of the RFID tag is depend on a direction of the RFID tag relative to that of the electromagnetic wave transmission sheet serving as an antenna to result in detecting no presence of the RFID tag in case when the RFID tag is positioned in a certain direction above the electromagnetic wave transmission sheet.